Optically bistable switching devices have been developed in a class known as self electrooptic effect devices (SEED). See U.S. Pat. No. 4,546,244. In SEED devices, optical bistability relies on incorporating semiconductor material whose absorption increases with increased excitation of the incorporated material.
An optically bistable SEED device generally comprises the interconnection of a p-i-n diode having an intrinsic quantum well region, an electrical or electronic load, and a bias voltage supply. The load and bias voltage supply are arranged in a feedback loop around the diode usually in a reverse bias configuration. When an electric field is applied perpendicular to the quantum well layers to permit electroabsorption by the quantumconfined Stark effect (QCSE), the absorption band edge including any sharp exciton resonance peaks is shifted to lower photon energies to achieve transmission changes of approximately 50%. Because the absorption edge is shifted toward lower photon energy under applied field conditions, the device is called a "red shift" device owing to the lower photon energy of red light in the visible light spectrum. Interband transitions give rise to a substantial amount of absorption for the biased SEED device. In general, the contrast between the low and high absorption states of the device is sufficient to permit realization of useful devices for modulation and the like.
With low optical power incident on the SEED device, nearly all the bias voltage is dropped across the diode because there is negligible, if any, photocurrent. The wavelength of light incident on the photodiode is carefully selected to be at or near the exciton resonance wavelength at zero applied field for peak or maximum absorption of the light. As incident light impinges on the reverse biased p-i-n diode, an increasing photocurrent is generated to, in turn, reduce the voltage across the diode by increasing the voltage drop across the load. The reduced voltage permits increased absorption to occur as the exciton resonance peak shifts back toward its zero applied field wavelength. Increased absorption provides a further increased photocurrent which, under proper regenerative feedback conditions, yields device switching.
Optically bistable SEED devices have been developed and reported in the prior art to operate in accordance with the principles set forth above. These devices have exhibited room temperature operation at high speed and low switching energy despite the lack of a resonant optical cavity which is commonly used to reduce switching energy. Moreover, such devices have been characterized by quantum well regions employing symmetric quantum wells. Symmetric quantum wells are known to produce the desired band edge absorption shift (a red shift) under applied field conditions so that increased absorption of incident light leads to switching necessary for bistable device operation.